In an optical communication system, it is generally necessary to couple an optical fiber to an opto-electronic transmitter, receiver or transceiver device and to, in turn, couple the device to an electronic system such as a switching system or processing system. These connections can be facilitated by modularizing the device. Such optical modules include a housing in which are mounted opto-electronic elements, optical elements, and electronic elements. In a transceiver module, the opto-electronic elements commonly include one or more light sources, such as lasers, and one or more light detectors, such as photodiodes. The optical elements commonly include lenses and, in modules in which the optical paths are not linear, reflectors that redirect the optical beams. Electronic elements commonly include digital signal driver circuits for driving the lasers or other light sources and digital signal receiver circuits for processing the output of photodiodes or other light detectors.
Various optical transceiver module configurations are known. For example, a configuration commonly referred to as “Small Form Factor Pluggable” or SFP refers to a transceiver module having an elongated housing with a rectangular cross-sectional shape, where the rear of the module has an electrical connector that plugs into a bay of a front-panel cage, and the front of the module has an optical fiber cable extending from it or an optical connector that accepts an optical fiber plug.
Accurate alignment among optical fibers, opto-electronic elements, and optical elements is important for proper operation of an optical communication module. Three methods for achieving such alignment are known: active alignment, visual alignment, and passive alignment. In active alignment, a light source is activated, and the signal coupling between the light source and target (i.e., photodiode or optical fiber) is electronically monitored while repositioning the elements with respect to each other until a measured signal indicates maximum coupling efficiency. Active alignment is generally a tedious and uneconomical process because it involves a closed-loop control system, including a set of actuators, an efficient peak search algorithm, and attendant instrumentation.
Visual alignment also functions as a closed loop system but relies on visual cues, such as fiducials or position of the light beam (monitored through an infrared camera), instead of monitoring the magnitude output of the light source. The primary drawbacks to visual alignment are that capital equipment costs escalate rapidly with required placement accuracy, and the throughput can be comparable to that of an active alignment system.
Passive kinematic alignment involves mating elements through accurate physical features. A common example of this is placing a fiber into a silicon submount with an etched V-shaped groove. As silicon is a rigid material in which a V-shaped groove can be very accurately formed by etching, the fiber diameter and accurate dimensions of the V-shaped groove allow for very accurate positional control of the fiber.
A variant of passive alignment is optical self-alignment, in which a force inherent to the system pulls the elements together into proper alignment. An example of optical self-alignment would be the use of surface tension of solder to align a die-attach component such as a laser.
The primary advantages of using passive alignment techniques are the reduction in system and equipment investment and a general reduction in process complexity. The primary obstacle is that the inherent part (e.g., a silicon fiber submount) costs quickly escalate as the required accuracy of part features increases.